1. Field of the Invention
The present invention relates to an image forming apparatus for forming a two-dimensional image in which the pixels are arranged in the main scanning direction and the sub-scanning direction which is perpendicular to the main scanning direction. In particular, the present invention relates to an image forming apparatus which comprises a plurality of laser elements arranged in the sub-scanning direction and which can form an image simultaneously on a plurality of lines in the main scanning direction by simultaneously scanning the image in the main scanning direction with a plurality of laser lights emitted from a plurality of laser elements.
2. Description of Related Art
In an image forming apparatus using an electrophotographic process, in order to obtain a high resolution image and to realize the high speed processing, there is an image forming apparatus for forming an image simultaneously on a plurality of lines by simultaneously scanning an image by using a plurality of laser lights.
In such an image forming apparatus, for example, there is an image forming apparatus which changes the resolution in the sub-scanning direction of the image to be formed by selectively using a plurality of laser lights, and which can adjust the size in the sub-scanning direction of the image (See Japanese Patent Application Publication No. 2009-34991). The above image forming apparatus comprises a laser light source which comprises a plurality of laser elements and which can scan an image in parallel with a plurality of lines in the sub-scanning direction at the second resolution which is higher than the first resolution of the image formed in the main scanning direction. The magnification of the image in the sub-scanning direction is set according to the first resolution, the second resolution and the image size of the image to be formed, and some of a plurality of laser elements are selected and used according to the set magnification.
Further, in case that the integration degree of the surface emitting laser element (VCSEL) in the two-dimensional surface emitting laser array (two-dimensional VCSEL array) is increased, the thermal interference is caused between the adjacent laser elements (VCSEL). In order to suppress the thermal interference, after as the first main scanning in the direction perpendicular to the first base line, the VCSELs provided on the first base line and the second base line are emitted so as not to simultaneously emit the adjacent VCSELs on the first base line and so as not to simultaneously emit the adjacent VCSELs on the second base line, as the second main scanning in the direction perpendicular to the first base line, the VCSELs provided on the first base line and the second base line are emitted among the VCSELs which are not emitted in the first main scanning, so as not to simultaneously emit the adjacent VCSELs on the first base line and so as not to simultaneously emit the adjacent VCSELs on the second base line.
In an image forming apparatus using the laser elements, an LD control unit comprising laser elements and the drive circuit thereof is arranged near the laser scanning unit (or so as to be integrated with the laser scanning unit) in view of the scanning optional system for the laser light and the other mechanical request. On the other hand, an image processing unit for correcting the inclination of the image or executing the fine variable magnification for the image by processing the image data is provided apart from the LD control unit because the space is limited in the apparatus. The image processing unit transmits a signal for controlling the turning on/off of the laser elements (laser diode: LDs) provided in the LD control unit to the LD control unit in synchronization with the scanning for forming an image by using the laser lights. The above signal is a pulse width modulation signal corresponding to the value of the image data in each pixel. Therefore, the transmission path for the above signal must be high quality in order to transmit the signal at a low noise, and becomes expensive.
In case of the image forming apparatus for simultaneously forming an image on a plurality of lines by scanning the image using a plurality of laser lights emitted from a plurality of laser elements, as shown in FIG. 18, the PWM modulation circuits 121 are required for the number of the laser elements in the image processing unit 120. Further, in order to transmit the pulse width modulation signals generated by the image processing unit 120 to the LD control unit 110 at a low noise, the transmission paths having the high quality are required for the number of the laser elements. In case that the number of the laser elements is increased (for example, 32, 64 or more, or the like), the required number of the PWM modulation circuits and the required number of the transmission paths are increased. Therefore, the cost of the apparatus is increased and it is difficult to secure the space for arranging the transmission paths.
Each of Japanese Patent Application Publication No. 2009-34991 and Japanese Patent Application Publication No. 2007-109929 relates to the technology for selectively driving a plurality of emitting elements. However, the technology for reducing the number of the PWM modulation circuits or the number of the transmission paths for the pulse width modulation signals is not disclosed. Further, in each of the above technologies, in case that some of a plurality of emitting elements are selectively used, the deterioration of the resolution and/or the deterioration of the productivity are caused.